Calcium Orthophosphates as Bioceramics - Calcium Orthophosphates: Applications in Nature, Biology and Medicine - page 345

reaches the maximum and rapidly drops with decreasing grain size.
For example, Halouani et al. investigated a fracture toughness of
pure hot pressed HA with grain sizes between 0.2 and 1.2 µm. They
found two distinct trends, where fracture toughness decreased with
increasing grain size above ~0.4 µm and subsequently decreased
with decreasing grain size. The maximum fracture toughness
measured was 1.20 ± 0.05 MPa·m

1/2

at ~0.4 µm [272]. Fracture
energy of HA bioceramics is in the range of 2.3-20 J/m , while the

Weibull modulus [303] is low (~5-12) in wet environments, which
means that HA behaves as a typical brittle ceramics and indicates
to a low reliability of HA implants [233]. Interestingly, but 3 peaks
of internal friction were found at temperatures about −40, 80 and
130°C for HA but no internal friction peaks were obtained for FA
in the measured temperature range; this effect was attributed to
the differences of F

2

− −

and OH positions in FA and HA, respectively

[304].
Bending, compressive and tensile strengths of dense HA
bioceramics are in the ranges of 38-250 MPa, 120-900 MPa and 38-
300 MPa, respectively. Similar values for porous HA bioceramics are
in the ranges of 2-11 MPa, 2-100 MPa and ~3 MPa, respectively [233].
These wide variations in the properties are due to both structural
variations (e.g , an influence of remaining microporosity, grain sizes,

presence of impurities, etc.) and manufacturing processes, as well as
they are caused by a statistical nature of the strength distribution.
Strength [305] was found to increase with Ca/P ratio increasing,
reaching the maximum value around Ca/P ~1.67 (stoichiometric
HA) and decreases suddenly when Ca/P > 1.67 [233]. Furthermore,
strength decreases almost exponentially with porosity increasing
[112, 113]. However, by changing the pore geometry, it is possible
to influence the strength of porous bioceramics. It is also worth
mentioning that porous HA bioceramics is considerably less fatigue
[306] resistant than dense ones. Both grain sizes and porosity are
reported to influence the fracture path, which itself has a little effect
on the fracture toughness of calcium orthophosphate bioceramics
[203, 307]. Furthermore, no obvious decrease in mechanical
properties was found after calcium orthophosphate bioceramics had
been aged in the various solutions during the different time periods
[308].
Young's (or elastic) modulus [309] of dense HA bioceramics is
in the range of 35-120 GPa, which is more or less similar to those

.